1. Field of the Invention
The present invention relates to an integrated interference scanning method, and in particular to an interference scanning method integrating the VSI and PSI measurements.
2. The Prior Arts
In the prior art, the interferometry is utilized to obtain the surface profile of the object to be measured through the interference fringes by making use of the light path difference and interference principle. In general, in terms of computation algorithm, the interferometry technology can be classified into: the Vertical Scanning Interferometry (VSI) and Phase Shifting Interferometry (PSI).
In the VSI scanning technology, the advantage of white light having short coherent wavelength is utilized to get rid of the interference noise, and derive the 3-D profile according to the wave-packet interference distribution. The measurement range of VSI algorithm does not have any limitations, and can be utilized to measure the object having large step height differences. However, if the resolution in the vertical direction is to be raised, then the step magnitude of scan in the vertical direction must be reduced. Therefore, the time spent on measurement will be increased accordingly. Besides, its major drawback is that the accuracy or precision of the measurement is not very satisfactory. In summary, the Vertical Scanning Interferometry (VSI) is suitable for the measurement environment with lower precision, or the object to be measured having rough surface and large step differences. On the other hand, PSI scanning technology can be used to restore the 3-D profile of the object in cooperation with various phase rebuilt technology. Compared with VSI, PSI may be used to provide better precision; however, the depth measurement range is limited by the measuring light wavelength and the 2 π ambiguity. Therefore, it is not suitable for measuring object having step difference greater than λ/4, such as the object to be measured having a large step height. Regarding this, a double-wavelength light wave phase-shift interferometry has been developed recently, which can be used to enlarge the measurement range of a large step height. However, when the wavelength of the coherent light wave is longer, it may cause the increase of noise due to self-interference, and this problem is difficult to solve with the present technology.
In this respect, some integrated measurement method is proposed to fully utilize the advantage and redress the shortcomings of the VSI and PSI measurements.
Since the zero optical path difference of VSI is calculated and obtained by utilizing the wave-packet information of the vertical scanning, every individual pixel is independent and is not affected by other pixels. However, in PSI, the height of said point is calculated and obtained by the phase change relative to the adjacent pixels. Yet, in the measurement utilizing PSI, if the light of wideband is utilized as the light source, then the spacings between the interference fringes (namely, the average wavelength) of the interference spectrum are practically affected by the numeral aperture (NA) of the optical system, thus resulting in the deviation between the average wavelength calculated from the spacing between the interference fringes and the actual average wavelength. Therefore, when merging the height information of VSI and PSI, their heights cannot be merged directly (since there are the inconsistence of height difference (intersection distance) and inclination (affected by the slope-NA value). For this reason, the conventional technology requires a single frequency light of known wavelength to eliminate such a deviation and achieve precise PSI measurement. Therefore, in this conventional scanning system, in addition to the wideband light source used for VSI measurement, another narrow-band light source is required for PSI measurement.
As such, in the conventional scanning system, both the wideband light source and narrow-band light source are provided. Thus, the VSI measurement is first performed utilizing the white light source, then the system is switched to a narrow-band light source by making use of a switching mechanism to perform PSI measurement of the object required, thereby realizing a two-stage measurement. However, in the implementation of this technology, two light sources are required to perform the VSI and PSI measurements respectively. Therefore, an additional mechanism is required to do the switching, hereby raising the cost of the system. Moreover, since a two-stage scanning is required in this conventional measurement, the vertical scan- image-fetching action has to be performed both in VSI and PSI, thus the measurement efficiency is adversely affected.
Therefore, in view of the shortcomings and drawbacks of the interference measurement system of the prior art, the research and development of an integrated interference scanning method, which is capable of providing much more simple, fast and accurate measurement, is the most urgent and important task in this field.